Aspects of the invention can relate to a projector which projects an image by use of an optical modulator, such as a liquid crystal panel or the like. There are related art projectors which color-synthesize image light of each color emitted from a liquid crystal panel of each color illuminated by illumination light of blue, red, and green, by a synthetic optical system having structure in which a pair of multi-layer films for blue reflection and for red reflection are arranged in the shape of an X character. See, for example, JP-A-64-11289, JP-A-8-114779, JP-A-2-245749, JP-A-2001-264522 and JP-A-2001-318221. In the related art projectors, blue image light is reflected by the multi layer film for blue reflection, red image light is reflected by the multi layer film for red reflection, and the both multi layer films transmit green image light, whereby superposition of the image light, that is, color synthesis is performed. See, for example, JP-A-64-11289, JP-A-8-114779, JP-A-2-245749, JP-A-2001-264522 and JP-A-2001-318221.
In a first projector of these related art projectors, in order to heighten efficiency of use for polarization light in a specified direction which is utilized for each color, of source light, a polarization rotary element which changes a polarization direction to 90° can be arranged between a liquid crystal panel arranged on a red or blue optical path and a synthetic dichroic prism arranged on an exit side of the light (JP-A-64-11289). Further, in a second related art projector, in order to heighten efficiency of use for polarization light in a specified direction which is utilized for each color, of source light, transmission characteristic of a dichroic prism for separation and transmission characteristic of a synthetic dichroic prism are adjusted to the polarized direction of each color light (JP-A-8-114779). Further, also in a third related art projector, in order to heighten efficiency of use for polarization light in a specified direction which is used for each color, of source light, transmission characteristic of a dichroic prism for separation and transmission characteristic of a synthetic dichroic prism are adjusted to the polarized direction of each color light (JP-A-2-245749). Further, in a fourth related art projector, in consideration with the influence of an incident angle to a synthetic dichroic prism on transmission characteristic, a half wavelength for S polarization of a multilayer film for red reflection, and a half wavelength for S polarization of a multilayer film for blue reflection are set in a predetermined range (JP-A-2001-264522). Further, in a fifth related art projector, by controlling the structure of a multilayer film for red or blue reflection incorporated into a synthetic dichroic prism, difference between an objected half wavelength for S polarization of the multi layer film and an obtained half wavelength for P polarization are reduced, and characteristic deterioration with change in an incident angle is prevented (JP-A-2001-318221).
Further, as another related art projector, there is a projector which separates light from a light source into three-color light, in which a light guide unit such as a relay optical system is arranged on a path of green illumination light thereby to compensate a difference in an optical path between the green light and the other two color-light (WO94/22042). In this projector, in a synthetic optical system, red image light is reflected by a multi layer film for red reflection, green image light is reflected by a multi layer film for green reflection, and the both multi layer films transmit blue image light, whereby color synthesis of an image is performed.
However, in case of the related art projector using the synthetic optical system including the multi-layer film for blue reflection and the multi layer film for red reflection, an illumination device for illuminating the liquid panel of each color is restricted to the constitution in which the green light is branched in the center. Namely, this synthetic optical system is restricted to optical-path arrangement in which the green image light is caused to go straight ahead and the blue and red image light are caused to couple to each other from both sides of the green optical path. Therefore, it is difficult to realize a projector in which on a green optical path a relay optical system that makes its relative optical path long is arranged.
Further, in the related art projector in which the relay optical system is arranged on the green optical path, a cross dichroic prism that is a synthetic optical system is composed of the multi layer film for blue or red reflection and the multilayer film for green reflection. However, regarding image light in a boundary wavelength of green and blue, it has been known that a ghost image is formed on a screen by multiple reflection in the both multi layer films.